Sources close to BK tell TMZ, the 19-year-old finally 'fessed up to her close buddies ... she and 22-year-old Nick Gordon are dating -- ten years after Whitney unofficially adopted the boy because he was having problems at home.

TMZ broke the story ... Bobbi's grandma Cissy Houston saw the video we posted -- showing Bobbi and Nick holding hands and kissing on a lunch run in Atlanta this week -- and told a close friend, "What [Bobbi and Nick] are doing is incestuous."

But Bobbi couldn't care less -- telling friends ... she doesn't give a damn about what Cissy thinks about her controversial relationship ... and because Nick's not her blood brother, she definitely doesn't consider it incest.

As one source put it -- Bobbi "can't please everyone, so she is going to please herself by doing what she wants to do for once."

As for the maybe engagement ring Bobbi was wearing -- we're told she's been mum on what it means ... but cryptically told friends, she and Nick ARE planning a trip to Vegas in the near future.